1. Field of the Invention
The present invention relates to compact dual-band antennas and to a method for adjusting the resonant frequency thereof. More particularly, the present invention relates to a dual-band antenna for use in on-vehicle communication devices, capable of transmitting and receiving signal waves in two frequency bands, and to a method for adjusting the resonant frequency of the dual-band antenna.
2. Description of the Related Art
An inverted-F antenna has been used for resonance in two frequencies. One type of known dual-band inverted-F antenna has a radiation conductor plate with a cutout portion that allows for resonance at two frequencies, i.e., high and low frequencies. Such an antenna is shown in, for example, Japanese Unexamined Patent Application Publication No. 10-93332.
FIG. 6 is a perspective view of an inverted-F dual-band antenna 1 of the related art. In the dual-band antenna 1, a radiation conductor plate 2 has a rectangular cutout portion 4, and provides an L-shaped conductor strip 2a that is resonated at a first frequency f1 and a rectangular conductor strip 2b that is resonated at a second frequency f2 higher than the first frequency f1. One side edge of the radiation conductor plate 2 is continuously formed with a short-circuiting conductor plate 3. The short-circuiting conductor plate 3 is disposed in an upright position on a ground conductor plate 5 for short-circuiting between the radiation conductor plate 2 and the ground conductor plate 5. The radiation conductor plate 2 faces the ground conductor plate 5 with a predetermined distance therebetween. A feed pin 6 is soldered at a predetermined position of the radiation conductor plate 2. The feed pin 6 is connected with a feed circuit (not shown) not in contact with the ground conductor plate 5.
In the dual-band antenna 1 of the related art, the longitudinal length of the L-shaped conductor strip 2a is set to about a quarter of the resonance length λ1 corresponding to the first frequency f1, and the shorter longitudinal length of the rectangular conductor strip 2b is set to about a quarter of the resonance length λ2 corresponding to the second frequency f2, where λ2<λ1. When predetermined high-frequency power is supplied to the radiation conductor plate 2 via the feed pin 6, the conductor strips 2a and 2b are resonated at different frequencies, and signal waves in two frequency bands, i.e., high and low frequency bands, are transmitted and received.
In dual-band antennas that can be resonated at two frequencies, i.e., high and low frequencies, it is necessary to check whether or not a desired resonant frequency is obtained before the antennas are sold. In most cases, the resonant frequency for the low frequency band (low band) needs to be finely adjusted because, in antenna devices, generally, the lower the frequency, the narrower the bandwidth at which the antenna devices can be resonated.
In the dual-band antenna 1 of the related art shown in FIG. 6, since the radiation conductor plate 2 functions as both low-band and high-band antennas, it is not easy to adjust the resonant frequency for either band. For example, if a portion of the L-shaped conductor strip 2a for the low band is cut out to finely adjust the resonant frequency (i.e., the first frequency f1), the resonant frequency for the high band (i.e., the second frequency f2) is easily affected. Thus, a careful and high-precision cutting operation is required for finely adjusting the resonant frequency of the L-shaped conductor strip 2a, leading to a complex frequency adjusting operation and high production cost.